I’ve just restored factory settings on my Mac Pro and when I now turn it back on a get a file symbol with a question mark an

Solution
i have that issue too about month ago. The problem is the filevault system on your mac. Your disk is encrypted by the filevault. Try to reinstall the os. and if u using the high sierra/sierra open the system preferences, then go to the security & privacy, after that look up to the filevault and turn off that. Or u can open terminal and paste this "sudo fdesetup disable" without ("). After that type your password and you're done.
i have that issue too about month ago. The problem is the filevault system on your mac. Your disk is encrypted by the filevault. Try to reinstall the os. and if u using the high sierra/sierra open the system preferences, then go to the security & privacy, after that look up to the filevault and turn off that. Or u can open terminal and paste this "sudo fdesetup disable" without ("). After that type your password and you're done.
 
Solution
Apple Cleaned my Mac Pro and brought it to Factory settings. My Iomega was working before I went for work on Laptop. Now when I attach the HDD OS X Base System comes up.